A major source of elemental sulfur is sour natural gas processing in which the H.sub.2 S and CO.sub.2 are preferentially removed from the gas by absorption in ethanolamine or other solvents. Upon saturation, the acid gas (containing H.sub.2 S and CO.sub.2) is stripped from the amine solution.
Conversion of the toxic hydrogen sulfide to elemental sulfur and water is achieved by the modified Claus process. According to this process, the hydrogen sulfide is first oxidized with a stoichiometric amount of air in a reaction furnace at approximately 1200.degree. C. Elemental sulfur, in an amount of 50-60% of the total sulfur content, is formed, along with water vapor, sulfur dioxide, carbonyl sulfide and carbon disulfide. The product stream is cooled to about 150.degree. C. so that the elemental sulfur is condensed and can be removed. The remaining gas, having a stoichiometric ratio of H.sub.2 S and SO.sub.2 of 2:1, is then fed to a series of adiabatic Claus catalytic converters, where the Claus reaction (1) is practised. ##EQU1##
The Catalyst
Activated alumina catalyst, in the form of porous granules 4-10 mm. in diameter, is normally employed in the converters. The catalyst has a reported composition and surface area as follows:
TABLE I ______________________________________ Component Wt. % ______________________________________ SiO.sub.2 0.02 Fe.sub.2 O.sub.3 0.02 Na.sub.2 O 0.35 Loss on ignition 6.0 Al.sub.2 O.sub.3 93.6 Surface area 325 m.sup.2 /g ______________________________________
With use, the catalyst loses its effectiveness. This deterioration arises from the generation and deposition of aluminum sulfate, carbon, and sulfur on the catalyst. High temperature and elevated partial pressure of water in the Claus reactors favour sintering of the catalyst. As a result, the surface area and activity of the catalyst can be reduced significantly.
By way of comparison, an exemplary analysis for a poisoned and regenerated catalyst is:
TABLE II ______________________________________ Wt. % Component (Poisoned) (Regenerated) ______________________________________ Total sulfur as SO.sub.4.sup..dbd., % 3.2 1.1 Water soluble SO.sub.4.sup..dbd., % 2.2 Carbon, % 2.2 Trace Al.sub.2 O.sub.3, % 59.1 90.8 Surface area, M.sup.2 /g 88 158 Pore vol., ml/g &gt;0.1 0.4 Loss on ignition 20.2 7.3 ______________________________________
Regeneration of the Catalyst
To applicant's knowledge, there are only two techniques which have been developed for regenerating poisoned Claus catalyst.
The first such technique involves a series of steps carried out while the poisoned catalyst remains in place in the converters (in situ regeneration). In this process, the operating temperature in the converters is raised well above the sulfur dew point and maintained at that level for 24-36 hours while a dilute H.sub.2 S-SO.sub.2 stream is circulated through the reactor. Under these conditions sulfur is volatilized and removed.
The catalyst is then subjected to an oxidative burn-off using air, to remove carbon, at a temperature in the order of 450.degree.-550.degree. C. However, under this oxidative environment any residual sulfur adsorbed on the catalyst can lead to sulfate formation. To reduce the SO.sub.4.sup.32 content, a 4:1 H.sub.2 S/SO.sub.2 mixture is subsequently flowed through converters, to reduce the aluminum sulfate to aluminum sulfide.
This in situ regeneration can take up to two weeks to complete and the degree of regeneration is often unsatisfactory and unreliable.
The other regeneration technique referred to is described in my U.S. Pat. No. 4,183,823. This process involves removing the poisoned catalyst from the converters, regenerating it, and returning it to the converters. The regeneration procedure comprises:
(1) leaching the aluminum sulfate and other soluble impurities from the catalyst using hot water; PA1 (2) drying the catalyst; PA1 (3) subjecting the catalyst to an oxidative burn-off to remove the adsorbed sulfur and carbon; PA1 (4) and repeating the leach, if SO.sub.4.sup.32 content is high. PA1 (1) first subjecting the catalyst to an oxidative burn-off, to remove sulfur and carbon, followed by PA1 (2) contacting the catalyst with a base, such as NaOH, to enhance the activity of the catalyst and to reduce sulfate content.
The drying step is necessary, as the presence of water during the oxidative burn-off step results in enhanced sintering of the catalyst and loss of reactive surface area.
When this patented process was examined for commercial application, it was found that the leaching and drying steps were expensive and detracted from the commercial viability of the process. Also, it was found that the activity of the regenerated catalyst was not consistent from one batch to another.